XJR15 compressed valve spring height

Can anyone tell me what the compressed valve spring height should be on an XJR15?
I’ve stripped the heads. Having previously measured the heights they’re all over the place! The shims under the spring seats are either 10thou thick or 5 thou thick. So, I assume there’s a 5thou tolerance?
It appears the compressed height on inlet and exhaust is different. As the measured figures I found were very different. Any help would be great.